There are at least four genera of SLM's: electro-optic, magneto-optic, liquid crystal and DMDs. The latter genus, DMDs, includes a micromechanical array of electronically addressable mirror elements. The mirror elements are reflectors each of which is individually movable. Each mirror is a light reflecting pixel and is capable of mechanical movement in response to an electrical input. Light incident on each mirror may be modulated in its direction (or phase) by reflection from each mirror. To date, DMD SLMs have found use in applications such as optical correlation, spectrum analysis, crossbar switching, frequency excision, display projection, printing and neural networks.
There are several species of the genus "DMD SLM", including elastomer type, membrane type as well as cantilever-beam, torsion-beam, and flexure-beam types. Selective control, or addressing, of the DMD SLM pixels has been achieved by electron-beam input, optically or, as preferred today, by circuitry integrated onto the DMD substrate.
Each pixel of the DMD SLM array reflects incident light along a path which depends on the position or orientation of the pixel. Typically, each mirror is movable or deflectable between a normal, first position or orientation and one or more second positions or orientations. In only one position, either the normal position or one of the second positions, a mirror directs the incident light along a selected path to a primary light-receiving site, for example, into an optical system and from there onto a viewing surface or a photosensitive drum. In all other pixel positions, incident light is not directed along the selected path to the primary site; rather, it is directed along another path to either a secondary site or to a "light sink" which absorbs or eliminates the light.
A DMD may take the form of a square, or nearly square, array of mirrors. In this event, the positions of the pixels, each of which is individually controllable by associated addressing facilities, may be altered to generate a video presentation. See commonly assigned U.S. Pat. Nos. 5,079,544; 5,061,049; 4,954,789; 4,728,185 and 3,600,798. See also U.S. Pat. Nos. 4,356,730; 4,229,732; 3,896,338 and 3,886,310. The mirror array may also take other forms, for example, that of a linear array or an array with many more columns than rows, the length of which is much larger than its width. In this latter event, the positions of the mirrors, as determined by their associated addressing facilities, may be altered so that the reflected light prints characters in quasi-line-at-a-time fashion on a photosensitive drum. See commonly assigned U.S. Pat. Nos. 5,101,236 and 5,04 1,851. In both events, and in other use environments, appropriate configurations of mirrors enable DMDs to modulate light in amplitude-dominant or phase-dominant modes.
It has been found convenient to produce integrated addressing circuits monolithically with the mirrors using conventional MOS processing techniques to form the addressing circuits in and on a substrate (e.g., silicon or GaAs) with the mirrors thereabove. The addressing circuits may be planarized and overlain by their respective mirrors to limit light penetration to the circuitry and to minimize the device size. Depending on the device type and the addressing voltages applied, the pixel may be addressed in analog, tristable, or bistable (binary) fashion.
A membrane type DMD SLM includes a metallized polymer membrane stretched over a spacer grid or other support structure. The openings in the grid define modulator cells or elements which comprise an address electrode and a portion of the polymer membrane supported by the spacer grid. The spacer grid effects an air gap or separation between segments of the membrane and the corresponding underlying addressing electrodes. When an address electrode of an address circuit is energized, by applying a bias voltage to the address electrode, the normally flat related membrane segment is curvilinearly deformed out of its normal, unstretched, planar position by electrostatic attraction between the membrane and the address electrode, and into the air gap where it now acts as a miniature spherical mirror. This deformation stores potential energy in the deformed membrane. When the address electrode is de-energized, the potential energy stored by the membrane returns the membrane to its normal flat position. Incident light reflected by each miniature spherical mirror is concentrated into a relatively narrow cone that is rotationally symmetric about the specularly reflected light. The pixel array can, therefore, be associated with a Schlieren stop, which comprises a single, central obscuration having a position and size to block the image of the light source that results from specular reflection by flat or un-modulated pixels. Modulated or spherically deformed pixels direct a circular patch of light onto the plane of the stop; the patch is centered on, but is larger than, the stop's central obscuration and, therefore, traverses a selected direction and reaches a selected site.
Membrane DMDs have also been produced by forming an array of relatively thick, separated, fiat mirrors supported on a relatively thin polymer membrane above a silicon or other substrate. The underlying addressing circuits formed on and in the substrate are separated by air gaps from their associated pixels when the latter reside in their normal positions. When an addressing circuit is appropriately energized, its mirror or pixel is displaced or deflected toward the substrate by electrostatic attraction. The mirrors remain flat while the membrane immediately surrounding them stretches to permit the mirrors to deflect up-and-down in piston-like fashion. The resultant displacement pattern produces a corresponding phase modulation pattern for the reflected light. This pattern may be converted into analog intensity variations by Schlieren projection techniques or used as the input transducer for an optical information processor. Further information on membrane type DMDs may be obtained from commonly assigned U.S. Pat. No. 4,441,791.
Beam-type DMDs each comprise a relatively thick (for rigidity) mirror or metal member supported by one or more relatively thin (for compliance), integral beams or springs. Each mirror and its beam(s) is structurally supported above and separated from its associated addressing circuit, and an address or control electrode which is a part of the addressing circuit, by a spacer or support post which supports the beam.
In the absence of a deflecting force applied to the mirror or metal member, the beam maintains the mirror in a normal, generally horizontal position parallel to the substrate. When the address or control electrode is energized by having a voltage applied thereto by the addressing circuitry, a portion of the mirror aligned with the electrode along the lines of the resulting electrostatic field is electrostatically attracted toward the electrode. Cantilever and/or torsional bending occurs preferentially at the thin beam(s). Such bending stores potential energy in the beam(s) associated with a deflected mirror. The stored potential energy, which tends to return the mirror to its normal position, is effective to return the mirror when the control or address electrode no longer attracts it.
While the present invention may prove useful relative to all types of SLM's it finds particular utility with beam-type DMDs. Improper operation of these types of DMD's typically involves one or more mirrors of the array becoming "stuck" and remaining in a fixed position regardless of the operation of the applicable addressing circuits. A mirror may become stuck in either the "on" position, the position whereat incident light is reflected onto a viewing surface, or an "off" position, a position whereat incident light is not reflected onto the viewing surface. Pursuant to the foregoing nomenclature, a mirror or pixel which is stuck in the "on" position will continuously reflect light to the viewing surface as a continuous "bright spot," even when the pixel should be "off", i.e. should be transmitting no light to the viewing surface. Moreover, a mirror or pixel which is stuck in the "off" position will not reflect light to the viewing surface, resulting in a continuous "dark spot," even when the pixel should be "on" and transmitting light to the viewing surface. These undesired, improper, bright and dark spots are referred to herein as "defects."
Detects in DMD displays are, as a minimum, annoying to the viewer. They produce ongoing light or dark regions which persist regardless of visual changes occurring to the remainder of the display. At worst, such defects may prevent the display from presenting meaningful visual information. Due to the manner in which DMD SLM's are manufactured, it is not possible to gain access to pixels which are stuck for purposes of rendering them operative. Moreover, pixels which are properly operative when a DMD SLM display is first put into service may later become stuck. The reverse is also possible, that is, previously stuck mirrors may become unstuck. As a consequence, a method of reducing the visual impact of such defects on the viewer to eliminate annoyance and to ensure that meaningful information is presented, which method can be selectively effected on an as-needed basis, and is one object of the present invention.